The K-Ras 4A isoform promotes apoptosis but does not affect either lifespan or spontaneous tumor incidence in aging mice

Ras proteins function as molecular switches in signal transduction pathways, and, here, we examined the effects of the K-ras4A and 413 splice variants on cell function by comparing wild-type embryonic stem (ES) cells with K-ras(tm Delta 4A/tm Delta 4A) (exon 4A knock-out) ES cells which express K-ras4B only and K-ras(-/-) (exons 1 - 3 knock-out) ES cells which express neither splice variant, and intestinal epithelium from wild-type and K-ras(tm Delta 4A/tm Delta 4A) mice. RT-qPCR analysis found that K-ras4B expression was reduced in K-ras(tm Delta 4A/tm Delta 4A) ES cells but unaffected in small intestine. K-Ras deficiency did not affect ES cell growth, and K-Ras4A deficiency did not affect intestinal epithelial proliferation. K-ras(tm Delta 4A/tm Delta 4A) and K-ras(-/-) ES cells showed a reduced capacity for differentiation following LIF withdrawal, and K-ras(-/-) cells were least differentiated. K-Ras4A deficiency inhibited etoposide-induced apoptosis in ES cells and intestinal epithelial cells. However, K-ras(tm Delta 4A/tm Delta 4A) ES cells were more resistant to etoposide-induced apoptosis than K-ras(-/-) cells. The results indicate that (1) K-Ras4A promotes apoptosis while K-Ras4B inhibits it, and (2) K-Ras4B, and possibly K-Ras4A, promotes differentiation. The findings raise the possibility that alteration of the K-Ras4A4B isoform ratio modulates tumorigenesis by differentially affecting stem cell survival and/or differentiation. However, K-Ras4A deficiency did not affect life expectancy or spontaneous overall tumor incidence in aging mice. (c) 2005 Elsevier Inc. All rights reserved.